Stabilized ammonium nitrate

ABSTRACT

Solid ammonium nitrate particles are stabilized against thermal shock and cracking at phase transition temperatures by the inclusion of small but effective amounts of silicofluoride compound, a phosphate compound and a sulfate compound in combination, within the solid ammonium nitrate particles.

United States Patent Falck-Muss et al.

STABILIZED AMMONIUM NITRATE Inventors: Rolf Falck-Muss, Arzew, Algeria;Daniel J. Newman, Jackson Heights, N.Y.; Sydney Atkin, Springfield, NJ.

Assignee: Chemical Construction Corporation, New

York, NY.

Filed: Jan. 22, 1970 Appl. No.: 5,119

U.S. Cl ..23/103, 71/35 Int. Cl ..C0lc l/18 Field of Search ..23/l03;71/35; 252/385, 397

[ 51 Mar. 14, 1972 Primary ExaminerOscar R. Vertiz Assistant Examiner-G.O. Peters Attorney-J. L. Chaboty [5 7] ABSTRACT Solid ammonium nitrateparticles are stabilized against thermal shock and cracking at phasetransition temperatures by the inclusion of small but effective amountsof silicofluoride compound, a phosphate compound and a sulfate compoundin combination, within the solid ammonium nitrate particles.

9 Claims, No Drawings STABILIZED AMMONIUM NITRATE BACKGROUND OF THEINVENTION 1. Field of the Invention The invention relates to theproduction of solid discrete particles of ammonium nitrate, and theprevention of thermal shock and cracking due to internal stress in theparticles generated at phase transitions which occur due to temperaturechange.

2. Description of the Prior Art Numerous additive compounds have beensuggested, either singly or in combination, for inclusion in solidammonium nitrate particles, in order to prevent or alleviate the wellknown phenomenon of thermal shock and cracking of the particles whichreadily occurs in pure ammonium nitrate particles when the temperatureof the particles is raised or lowered through the phase transitiontemperatures, of which the 32 C. transition point is most significant inpractice. Among those compounds which have been suggested in the priorart are phosphates such as potassium metaphosphate, mono-and diammoniumphosphate, sulfates such as ammonium sulfate, potassium chloride,magnesium salts, calcium salts, sodium silicate, clays, nitrates such assodium nitrate, calcium nitrate and potassium nitrate, iron cyanides,copper oxides and boron compounds. These compounds and variouscombinations of compounds are proposed in US. Pat. Nos. 1,406,455;1,698,793; 1,868,890; 1,932,434; 1,939,165; 1,947,601; 1,966,947;2,124,332; 2,136,069; 2,657,977; 2,702,747; 2,879,133; 2,901,317;2,943,928; 2,957,763; 3,007,773; 3,018,164; 3,021,207; 3,026,193;3,030,179; 3,034,853;

3,034,858; 3,070,435; 3,116,108, 3,117,835; 3,148,945 and 3,317,276. Theinhibition of calcium nitrate formation in the production of nitrochalkby silicofluoride addition is disclosed in U.S. Pat. No. 3,351,454.

SUMMARY OF THE INVENTION In the present invention, it has beendetermined that an improved solid ammonium nitrate composition may beproduced in the form of discrete particles which resist thermal shockand cracking due to deformation at phase transition temperatures such as32C., by the inclusion of a synergistic combination of compounds intothe solid ammonium nitrate, preferably by the addition of small buteffective amounts of these compounds to molten ammonium nitrate prior tosolidification of the ammonium nitrate as discrete particles by prillingor the like. The molten ammonium nitrate is preferably initially in theform of a substantially anhydrous melt, however in some cases the meltmay contain water in an amount up to about 5 percent by weight. Theadditive compounds employed in combination in the present invention area silicofluoride compound, a phosphate compound and a sulfate compound.Numerous compounds within each of these categories may be effectivelyemployed to stabilize ammonium nin'ate. Among the feasiblesilicofluoride compounds are ammonium silicofluoride, sodiumsilicofluoride, potassium silicofluoride, magnesium silicofluoride andzinc silicofluoride, or mixtures of these compounds. Usable phosphatecompounds include monoammonium phosphate, diammonium phosphate,trisodium phosphate and mixed fertilizer compositions which includephosphatic fertilizer components. Among the many sulfate compounds whichmay be employed in the present invention are ammonium sulfate, sodiumsulfate, potassium sulfate and calcium sulfate. A preferred method ofadding a phosphate compound and a sulfate compound to the moltenammonium nitrate is by the addition of phosphoric acid and sulfuric acidto the molten ammonium nitrate, or to its precursor nitric acid,followed by in situ neutralization of the acids by addition of asuitable base or basic solution to the molten ammonium nitrate. Typicalfeasible basic materials for this purpose include ammonia, sodiumhydroxide, sodium carbonate potassium hydroxide and potassium carbonate,and these bases may by added either in the anhydrous state or as aqueoussolutions. The molten ammonium nitrate containing the additive compoundsof the present invention in combination is preferably solidified in theform of discrete particles by a prilling procedure, in which the moltenammonium nitrate is sprayed into an airstream, so that the resultingammonium nitrate droplets solidify in the airstream in the form ofspherical solid prills. The resultant prills preferably contain up toabout 0.5 percent by weight of each of the additive compounds, and inaccordance with the present invention the resulting prills are highlyresistant to thermal shock and cracking at the phase transitiontemperatures such as 32C. Alternative methods of solidifying the moltenammonium nitrate in the form of discrete particles after additivesaddition include granulation, crystallization, or by flowing the moltenammonium nitrate onto a chilled belt on which the molten materialsolidifies in the fonn of a thin sheet or film which is then broken upinto small crystals or particles.

The principal advantage of the present invention is that the resultingsolid ammonium nitrate composition is stabilized against thermal shockand cracking at phase transition temperatures, so that the product maybe safely stored for extended periods of time without the development ofa dusting or caking problem. Another advantage is that the production offines or powdered material during the preparation of the discrete solidparticles by prilling or the like is substantially reduced, so that therecycle of fines is reduced in the ammonium nitrate production facility,which reduces the capital and operating costs such as by decreasingstream consumption for concentration of remelt solutions. A furtheradvantage is that the additive compounds of the present invention arerelatively inexpensive, and in most cases these additives are readilyavailable at fertilizer complexes or production sites, especially whenphosphatic fertilizers are concomitantly produced from phosphate rock,phosphatic shales or the like. Silicofluorides are generally producedand recovered as byproducts in the processing of phosphatic rawmaterials to produce phosphate products, as for example in theproduction of phosphoric acid as described in US. Pat. No. 2,905,535. Anadditional advantage is that the additive compounds of the presentinvention can be disseminated and dissolved into the molten ammoniumnitrate, and the provision of special or ancillary mixing equipment orapparatus is minimized.

It is an object of the present invention to provide an improved solidammonium nitrate product.

Another object is to prevent thennal shock and cracking of solidammonium nitrate particles at phase transition temperatures, by theaddition of a synergistic combination of additive compounds to theammonium nitrate.

A further object is to produce improved ammonium nitrate prills whichare resistant to thermal shock and cracking at phase transitiontemperatures, and which may be safely stored for extended periods oftime without dusting or caking.

An additional object is to reduce the amount of fines and powder whichare formed when molten ammonium nitrate is converted to discrete solidparticles by prilling or the like.

An object is to provide an improved method of stabilizing solid ammoniumnitrate particles against thermal shock and cracking at phase transitiontemperatures, by employing a combination of additive compounds in theammonium nitrate including a silicofluoride compound, a phosphatecompound and a sulfate compound.

These and other objects and advantages of the present invention willbecome evident from the description which follows.

DESCRIPTION OF PREFERRED EMBODIMENTS Ammonium nitrate crystals or prillswithout additives undergo volume expansion and contraction as they passthrough the phase transition temperatures such as 32 and 83C., causingthe prills to break down into fine particles or dust. The effect ofvarious additives including the additives combination of the presentinvention was investigated on a laboratory scale, followed by furthertesting in a commercial facility,

which demonstrated the improved results obtained by the presentinvention.

In all cases, the prills were subjected to a number of temperaturechanges through a range which included a phase transition temperature,and the percentage of prills cracked from thermal shock at variousnumbers of transition was determined. Following are the results obtainedin the laboratory scale and commercial plant facility tests.

EXAMPLE I. LABORATORY SCALE TESTS The ammonium nitrate prills producedin the laboratory were obtained by spraying a melt consisting of 99.5percent ammonium nitrate plus additive from a gun containing 0.029 inchholes using about 10 p.s.i.g. nitrogen pressure. In order to obtain morefailing height, the spray was arched from the top of a 27-feet-highplatform toward the ceiling and allowed to fall to the floor. The prillsproduced in this manner contained considerable irregularly shaped prillsand were weaker than prills produced in a commercial plant, butotherwise had characteristics similar to commercial prills.

TABLE I.

TYPICAL SCREEN ANALYSIS OF LABORATORY PRILLS Mesh Size By Weight .+6mesh 0.0 6+8 mesh 9.! 8+l0 mesh 33.3 -l+ l4 mesh 43.3 l4 mesh 14.3

For the phase transition tests, only 8 0 mesh prills were used. Twodifferent series of test were made to determine the resistance ofammonium nitrate prills produced without and with various additives, tothermal shock and cracking due to repeated passage through transitiontemperatures. In the first series of tests, prills were heated in anoven at 42C. for 2 hours and then allowed to cool to room temperature at23C. for 2 hours. The temperature changes thus provided for a transit ofthe phase transition temperature of 32C. Alter each cycle, thepercentage of unbroken prills was recorded. In the second series oftests, the prills were cycled between 91 and 42C. at 2 hour internals,to provide for a transit of the phase transition temperature of 83C.Following is data relative to the inclusion of additives in the prills.

TABLEII ADDITIVE INCLUSION IN PRILLS Run Nos. Additive (S) Added ByWeight I and I4 none 0 2 and 15 ammonium sulfate 0.2 2 and 15 diammoniumphosphate 0.2 2 and 15 sodium silicofluoride 0.2 3 and I6 diammoniurnphosphate 0.6 4 and 17 ammonium sulfate 0.6 5 and I8 ammonium sulfate0.3 5 and I8 diammonium phosphate 0.3 6 and 19 sodium silicol'luoride0.6 7 and 20 ammonium silicofluoride 0.6 8 and 2l potassiummetaphosphate 0.6 9 and 22 potassium metaphosphate 0.3 9 and 22 sodiumsilicofluoride 0.3 10 and 23 boric acid 0.6 l l and 24 boric acid 0.3 IIand 24 sodium silicofluoride 0.3 l2 and 25 aluminum phosphate 0.6 If!and 26 calcium carbonate 0.3 l3 and 26 ammonium sulfate 0.3

Number of transitions Percent prills cracked trom thermal shock (42O.-23 0. cycle) TABLE IV.-EFFECT OF ADDITIVES ON PRILL BREAKA GE FROMTHERMAL SHOCK AT 83 C. PHASE TRANSITION Number 0! transitions Percentprills cracked lrom thermal shock (91 C.42 0. cycle) From the results inTable IV, it is evident that the combination of additives of the presentinvention, as provided in Run No. 15, provided complete stabilization ofthe ammonium nitrate and attained the greatest improvement of all of theadditives or additive combinations tested, as compared to Run No. 14 inwhich no additive was present in the ammonium nitrate.

EXAMPLE I]. COMMERCIAL PLANT TESTS Test were conducted at a commercialammonium nitrate production facility to confirm the laboratory resultsof Example I. The facility was operated at a production rate of 300tons/day of prilled ammonium nitrate. Initial prilled ammonium nitratesamples were taken without the addition of stabilizer, and furthersamples were taken after the stabilizer of the present invention wasadded to the molten ammonium nitrate prior to prilling the melt. Sampleswere taken at the prill cooler outlet, which discharged prills withoutstabilizer addition at a temperature of 50C. With the addition ofstabilizer, prill cooler exit temperature was reduced to 38 or 29C. asindicated infra. The stabilizer was an aqueous solution made at the siteand consisting (by weight) of 75.4 percent water and 8.2 percent each ofsodium silicofluoride, ammonium sulfate and l8-460 fertilizer, thelatter component consisting principally of diammonium phosphate. Thefertilizer was dissolved in water, decanted and screened to remove sandand other insolubies. The stabilizer was constantly agitated and held at66C. by steam coils.

After taking dry run samples without additive addition, the

prill breakage from thermal shock at the 32 C. transition point.

TABLE V ADDITIVE INCLUSION 1N PRILLS (BY ANALYSIS) Percent By Weight ofAdditive D' A m Run Nov Phosphate Sulfate Silicofluoride TABLE VI PLANTOPERATING CONDITION Prill Cooler Run No. Exit Temperature TABLEVIL-EFFECT 0F STABILIZER ON PRILL BREAK- I%IiIOROM THERMAL SHOCK AT 320. PHASE TRAN- N umber of transitions Percent prills cracked fromthermal shock (42 C.23 0. cycle) The greatly improved results obtainedin Run Nos. 2 and 3 due to stabilizer addition, as compared to Run No. lin which no stabilizer additives were provided, clearly indicate thecommercial advantages of the invention in stabilizing ammonium nitrateagainst thermal shock at phase transitions. It should be noted thatwithout the additive, prills cannot be cooled below 50C. without greatlyexcessive breakage.

We claim:

1. A method of stabilizing solid ammonium nitrate particles againstthermal shock at phase transition temperatures which comprises adding asmall but effective amount of a silicofluoride compound, a phosphatecompound and a sulfate compound to molten ammonium nitrate, whereby saidcompounds are distributed in said molten ammonium nitrate, andsolidifying said molten ammonium nitrate in the form of dis creteparticles, said solid ammonium nitrate particles containing saidcompounds in sufficient proportion to render said particles resistant tocracking from thermal shock at phase transition temperatures.

2. The method of claim 1, in which said molten ammonium nitrate containswater in an amount up to about 5 percent by weight.

3. The method of claim 1, in which said silicofluoride compound isselected from the group consisting of ammonium silicofluoride, sodiumsilicofluoride, potassium silicofluoride, magnesium silicofluoride andzinc silicofluoride.

4. The method of claim 1, in which said phosphate compound is selectedfrom the group consisting of monoammonium phosphate, diammoniumphosphate and trisodium phosphate.

5. The method of claim 1, in which said sulfate compound is selectedfrom the group consisting of ammonium sulfate, sodium sulfate, potassiumsulfate and calcium sulfate.

6. The method of claim 1, in which said phosphate compound and saidsulfate compound are added to said molten ammonium nitrate by addingphosphoric acid and sulfuric acid to said molten ammonium nitrate duringproduction of the ammonium nitrate, and neutralizing said acids withinsaid molten ammonium nitrate by the addition of a base selected from thegroup consisting of ammonia, sodium hydroxide, sodium carbonate,potassium hydroxide and potassium carbonate.

7. The method of claim 1, in which said molten ammonium nitratecontaining said compounds is solidified in the form of discreteparticles by spraying said molten ammonium nitrate into an air stream,whereby said molten ammonium nitrate flows downwards through said airstream as discrete droplets which solidify in the form of prills.

8. The method of claim 1, in which each of said compounds is added tosaid molten ammonium nitrate in a proportion up to about 0.5 percent byweight of said molten ammonium nitrate.

9. A solid ammonium nitrate composition stabilized against thermal shockat phase transition temperatures which comprises ammonium nitratecontaining small but effective amounts of a silicofluoride compound, aphosphate compound and a sulfate compound.

Patent No 3649173 Dated March 14, 1972 Inventor(s) R. Falck-Muus, D. J.Newman, S. Atkin It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Col. 2 line 28, read "steam" instead of "stream". Also line 71, read"32C" instead of "32".

Col. 3 line- 6, read "transitions" instead of "transition". Also line46, read "91C" instead of "91". Also line 47, read "intervals" insteadof "internals".

Claim 1 line 3 (col. 6 line 7) after "adding", delete "a" and read "thecombination of". Also read "amounts" instead of "amount", Also read "atleast about 0.12% by weight of" after "of",

Claim 9 line 4 (col, 6 line 52) read "at least about 0.12% by weight of"after "of" Signed and sealed this 29th day of August 1972.

(SEAL) Attest:

EDWARD MJLETGHER,JR. ROBERT GOTTSCHALK Attosting Officer Commissioner ofPatents ORM FO-lOSO (10-69) USCOMM-DC 60376-7 69 u 5. GOVERNMENTPRINTING OFFICE: I969 0366-334

2. The method of claim 1, in which said molten ammonium nitrate containswater in an amount up to about 5 percent by weight.
 3. The method ofclaim 1, in which said silicofluoride compound is selected from thegroup consisting of ammonium silicofluoride, sodium silicofluoride,potassium silicofluoride, magnesium silicofluoride and zincsilicofluoride.
 4. The method of claim 1, in which said phosphatecompound is selected from the group consisting of monoammoniumphosphate, diammonium phosphate and trisodium phosphate.
 5. The methodof claim 1, in which said sulfate compound is selected from the groupconsisting of ammonium sulfate, sodium sulfate, potassium sulfate andcalcium sulfate.
 6. The method of claim 1, in which said phosphatecompound and said sulfate compound are added to said molten ammoniumnitrate by adding phosphoric acid and sulfuric acid to said moltenammonium nitrate during production of the ammonium nitrate, andneutralizing said acids within said molten ammonium nitrate by theaddition of a base selected from the group consisting of ammonia, sodiumhydroxide, sodium carbonate, potassium hydroxide and potassiumcarbonate.
 7. The method of claim 1, in which said molten ammoniumnitrate containing said compounds is solidified in the form of discreteparticles by spraying said molten ammonium nitrate into an air stream,whereby said molten ammonium nitrate flows downwards through said airstream as discrete droplets which solidify in the form of prills.
 8. Themethod of claim 1, in which each of said compounds is added to saidmolten ammonium nitrate in a proportion up to about 0.5 percent byweight of said molten ammonium nitrate.
 9. A solid ammonium nitratecomposition stabilized against thermal shock at phase transitiontemperatures which comprises ammonium nitrate containing small buteffective amounts of a silicofluoride compound, a phosphate compound anda sulfate compound.